Recently, energy saving has been desired for preventing global warming as an important global environmental issue, and the energy saving of a household appliance has been also promoted. Especially, a refrigerator-freezer requires a heat insulator having high insulating property in terms of efficient use of cold.
As a heat insulator of high performance, a vacuum heat insulator is used, which has a core formed of a porous body and an enveloping member. The core is covered by the enveloping member, inside of which is evacuated, and sealed. As the core of the vacuum heat insulator, generally, powder material, fiber material, or communicated foam is used. However, a vacuum heat insulator of higher insulation performance is required.
The heat transfer mechanism of the heat insulator is generally produced by heat conduction, radiation, or convection of a solid component and a gas component. Since the inside of the enveloping member is evacuated in the vacuum heat insulator, the heat conduction and convection of the gas component produce only a little effect on the heat transfer mechanism. When the heat transfer mechanism is used in a region of normal temperature or lower, the radiation also hardly contributes to it. It is therefore important to suppress the heat conduction of the solid component in the vacuum heat insulator that is applied to a refrigerator-freezer at normal temperature or lower. Thus, as the core for the vacuum heat insulator of high insulation performance, various fiber materials are reported.
Japanese Translation of PCT Publication No. H11-506708, for example, discloses a vacuum heat insulator having a core where thermoplastic inorganic binder material such as low-melting glass composition or boric acid is dispersed in the whole fibrous material. FIG. 7 is an enlarged schematic diagram showing an intersection point of cores used in this vacuum heat insulator. Two adjacent glass fibers 71 and 72 form binding part 74 at intersection point 73 using the inorganic binder material. Individual fibers of a fiber assembly are thus integrated. Examples of such products include a blanket made of electrical insulation material, a mat made of electrical insulation material, and a heat insulator. The inorganic binder material generates few gas components under vacuum in the enveloping member, unlike a general-purpose resin binder. The degradation over time in the heat insulation performance of the inorganic binder material is thus small.
In the structure described above, the binder bound at the intersection point of the inorganic fibers works as a binding material. The solidifying binder works as a thermal cross-link in binding part 74, thereby increasing the heat conduction in the heat insulation direction. In other words, the heat conductivity of the vacuum heat insulator is higher than that of a core that is made of a fiber body having no binding region of a binder or an eluting component.
In the fiber body having no binding region of a binder or an eluting component, the heat conductivity of the solid component is low. However, the fiber body lies in a bulky cotton state, and hence is extremely difficult to handle. When this fiber body is used as the core of the vacuum heat insulator, the external surface property is damaged by compression by atmosphere.